ATLb2.00/topo/README.topo.grid:

README.topo.grid		this file
regional.grid.[ab]		HYCOM grid location file
regional.grid.com		script to create regional.grid.[ab]
regional.grid.log		From csh regional.grid.com >& regional.grid.log

All HYCOM pre and post-processing programs read regional.grid.b at 
run-time to get idm and jdm for the particular region being processed.  
So regional.grid.com must be run first to generate regional.grid.[ab].
The source code is domain-independent (and therefore in ../../ALL/topo).

In this case the grid is "mercator", i.e. constant longitudinal grid 
spacing in degrees but latitudinal grid spacing in degrees varying with
cos(latitude) to give a square grid cell in meters.  The method of 
specifying the grid location is as used previously by MICOM and earlier
versions of HYCOM:

  0     'mapflg' = map flag (0=mercator,2=uniform,4=f-plane)
  1.0   'pntlon' = longitudinal reference grid point on pressure grid
263.0   'reflon' = longitude of reference grid point on pressure grid
  2.0   'grdlon' = longitudinal grid size (degrees)
 11.0   'pntlat' = latitudinal  reference grid point on pressure grid
  0.0   'reflat' = latitude of  reference grid point on pressure grid
  2.0   'grdlat' = latitudinal  grid size at the equator (degrees)

Grid array point (pntlon,pntlat) is at lon,lat location (reflon,reflat) 
with equatorial grid spacing of grdlon by grdlat.  For a mercator grid,
grdlon=grdlat and reflat is always zero (i.e. pntlat indicates the
array index of the equator, which need not be in 1:jdm).  The same 
program (ALL/topo/src/grid_mercator) can also produce cylindrical 
equidistant grids (constant latitudinal grid spacing, perhaps with 
grdlon .ne. grdlat) and and f-plane grids.  There are other programs 
in ALL/topo/src for non-constant/non-mercator latitudinal grids 
(grid_latitude) and global grids with an arctic dipole patch (grid_panam 
or grid_lpanam).

Any orthogonal curvilinear grid can be used, so if the above programs
don't meet your needs - just produce your own regional.grid.[ab] in the
right format.  The easiest way to get the format right is to use the
"zaio" routines to write the .a file, just as grid_mercator.f does.
The concents of the file are:

   57    'idm   ' = longitudinal array size
   52    'jdm   ' = latitudinal  array size
    0    'mapflg' = map flag (-1=unknown,0=mercator,2=uniform,4=f-plane)
plon:  min,max =       263.00000      375.00000
plat:  min,max =       -19.60579       63.11375
qlon:  min,max =       262.00000      374.00000
qlat:  min,max =       -20.54502       62.65800
ulon:  min,max =       262.00000      374.00000
ulat:  min,max =       -19.60579       63.11375
vlon:  min,max =       263.00000      375.00000
vlat:  min,max =       -20.54502       62.65800
pang:  min,max =         0.00000        0.00000
pscx:  min,max =    100565.21875   222389.87500
pscy:  min,max =    100571.95312   222378.59375
qscx:  min,max =    102139.75781   222356.01562
qscy:  min,max =    102146.85938   222344.73438
uscx:  min,max =    100565.21875   222389.87500
uscy:  min,max =    100571.95312   222378.59375
vscx:  min,max =    102139.75781   222356.01562
vscy:  min,max =    102146.85938   222344.73438
cori:  min,max =   -0.0000511824   0.0001295491
pasp:  min,max =         0.99993        1.00005

Three header lines in regional.grid.b identify the domain size and the
map projection (which is ignored for most purposes).  This is followed
by one line for each field in regional.grid.a:  (i) the longitude and
latitude of all four grids, (ii) the angle of the p grid w.r.t. a 
standard lat,lon grid, (iii) the grid spacing in meters of all four 
grids, (iv) the coriolis parameter (q-grid), and (v) the aspect ratio
of the p grid (pasx/pscy).